The barrel of a cannon extends along a barrel axis on which is centered a barrel tube surrounding and holding the cannon. The barrel can move limitedly axially in this tube, which itself can be pivoted on the carriage supporting it about orthogonal vertical and horizontal axes that intersect each other and the barrel axis at a common point. Pivoting of the barrel about the horizontal axis is for elevation aiming and about the vertical axis is for azimuth aiming. Shifting along the barrel axis is necessary for absorbing recoil and, on some guns, for reloading operations.
In a standard large-bore gun where the barrel shifts axially back with each shot, it is of course absolutely essential that this shifting be purely axial, wholly devoid of any radial component, or the shot will surely miss.
Accordingly it has been suggested to support the barrel in the barrel tube on slide bearings at the front or muzzle end and at the rear or breech end of the barrel tube. These bearings must very snugly engage the barrel to prevent any canting in the barrel tube. As a result they must be painstakingly fitted in place. Such a snug fit wears rapidly, especially as the barrel expands and contracts as it heats and cools during use, so that either shooting accuracy suffers, or the slide joint must be frequently adjusted and overhauled.
German patent document No. 3,013,988 filed Apr. 11, 1980 by R. Moller and G. Scharting suggests a system wherein the muzzle and breech ends of the barrel tube are each provided with a set of rollers. Normally three angularly equispaced rollers are used, all pivotal about axes lying in a common plane perpendicular to the barrel axis. The rollers are hyperboloidal, with radii of curvature equal to that of the barrel so they engage same in line contact at the above-mentioned plane. To compensate for wear one of the rollers of each set is carried in mounts that can be shifted radially of the barrel axis to move the respective roller radially in or out. In addition the mount for this movable roller is provided with springs which maintain a minimal radially inwardly directed pressure on the roller to keep the joint tight.
Such an arrangement has two main disadvantages. First of all the rollers normally slip at the start, since the barrel initially moves at great speed and the inertia of the rollers must be overcome. Thus there is a problem of sticking and wearing flats on the rollers. In addition the roller assembly is quite bulky, extending radially well beyond the barrel tube. It is therefore necessary to provide a mantlet or the like to shield this extra structure, which also can be a nuisance in general when handling and using the gun.